Pseudomonas aeruginosa is one of the chief causative agents of infection in severely granulocytopenic patients with acute leukemia and other malignancies. The prime objective of the study has been to monitor its epidemiology in cancer patients, to study its host-parasite interaction, and its antibiotic susceptibilities. Recovery rates are higher than last year. Serotype 12, (two patient recoveries in 10 years) was recovered from four patients. Serotypes of P. aeruginosa varied in their ability to activate the alternate complement pathway in nonimmune serum. Certain serotypes completely consumed complement by 1) the alternate pathway or 2) the classical pathway; other serotypes incompletely decomplemented this serum via the classical pathway. Recovery of nonfermentative gram negative bacilli from patients as well as their environment has increased; the majority include bacterial species that had been less frequently encountered in this patient population during the last ten years. P. paucimobilis, a recently described species, was also recovered.